The European Union has suspended funding for Cambodia's 2018 general election because the vote cannot be credible after the dissolution of the main opposition party, according to a letter sent to the national election committee on Tuesday.

The Cambodia National Rescue Party (CNRP) was dissolved by the country's highest court last month at the request of the government of Prime Minister Hun Sen after the arrest of opposition leader Kem Sokha for alleged treason.

"An electoral process from which the main opposition party has been arbitrarily excluded cannot be seen as legitimate," read the Dec. 12 letter reviewed by Reuters. "Under these circumstances, the European Union does not believe there is a possibility of a credible electoral process."

The EU and Japan are the biggest donors to Cambodia's election commission. Phay Siphan, a spokesman for the government, said it was able to hold the election with its own money.

"This is their will ... we have our own money," Phay Siphan told Reuters on Tuesday.

The United States last month said it would suspend funding for the election. It later said it would impose visa sanctions for people involved in the governments' actions to undermine democracy.

A crackdown by the ruling party has seen senior members of the opposition targeted in recent months. Kem Sokha, leader of the CNRP, was arrested for allegedly plotting to overthrow the government with U.S. help.

He has rejected the accusation as a political ploy.

The dissolution of the CNRP has been condemned by some Western countries as the most serious blow to democracy since an international peace deal and U.N.-run elections in the early 1990s ended decades of war and a Khmer Rouge genocide that killed at least 1.8 million Cambodians in the 1970s.

The EU in October warned Cambodia that it could face EU action over duty-free access it enjoys under a deal for some of the world's poorest countries if the human rights situation in the country deteriorates further.